A well known characteristic of the so called semi-conducting materials is the dependence of their physical properties with their temperature. This is particularly true for electronic devices and/or circuits of any type; discrete, integrated or hybrid, comprising at least a pn junction. By pn junction should be understood a structure of semiconducting materials constituted by two regions of semi-conducting materials, one of P type conductivity and the other of N type conductivity, with their respective electrical access or electrodes. Such temperature dependence remains irrespectively if the said pn junction is obtained by metallurgic doping of each one of the said P and N regions or some how it is just induced, as well as, of the primary intended uses of the said pn junction, such as; rectifier, photodiode, coherent or non coherent light emitting diode (of any wave length), solar cell, bipolar transistor, SCR's, etc. In general, the performance of electronic devices based on semi-conducting materials comprising or not a pn junction, is intrinsically dependent on the temperature at which the corresponding device is being operated. Because of this property, semi-conducting devices discrete or integrated are widely and extensively used as temperature sensors and, when calibrated, as thermometers. Particularly the pn junction or diode is widely used as a thermometer requiring, nevertheless, periodical calibrations.
The current flowing through a p-n junction, I(V) as a function of the forward bias voltage applied to the said p-n junction, V, and for values of the said applied forward bias voltage higher than 3kT, is given by the equationI(V)=IDS(T)×exp[q(V−RSI)/ηDkT]+ISR(T)×exp[q(V−RSI)/ηrkT]+RP/(V−IRS)  (1)where the IDS(T) pre-exponential term is the diode diffusion saturation current, which is a function of the temperature; T, at which the diode is operating, q is the electron charge, RS is the diode parasitic series resistance, ηD, is the ideality factor for the conduction current mechanism due to the minority carriers diffusion, k is the Boltzmann's constant, ISR(T), another pre-exponential term is the diode recombination saturation current, which is also a function of the temperature T at which the diode is functioning, ηr, is the ideality factor for the current due to the recombination process in the, so called, diode space charge region, and RP is the diode parasitic parallel resistance.
In the equation (1), the first term on the right hand side is due to the thermal diffusion of minority carriers in the neutral regions at each side of the p-n junction that have been injected across the junction as a result of the applied forward bias. The second term on the right hand side of the same equation (1), is due to the recombination of both types of carriers; electrons and holes, in the so called space charge region of the p-n junction, and the third term in the same equation (1) is due to a parasitic parallel resistance, the term (V−IRS) represents the portion of the applied external voltage V, that is effectively applied to the p-n junction and IRS is the portion of the said applied external voltage V, that drops across parasitic series resistance all sources together; leads, soldering and wires, necessaries to the bias and electrical measurement.
Usually, when the p-n junction diode is used as a temperature sensor is operated in a forward bias region where the first term of the equation (1) is dominant, that is to say, when the other terms can be neglected. Under such operating conditions when through a p-n junction diode flows a constant forward bias current; I0, the forward bias voltage externally applied to maintain that said constant current through the said pn junction diode varies with the temperature according to the equation (2), belowV(I0,T)=(ηDkT/q)×In(I0/IDS(T))+I0RS  (2)
Generally, both IDS and ISR depend on the p-n junction design and its manufacture technological process, RS and RP are, as well, dependent on the diode's technology and ηD and ηr should be equal, according to Schockley's p-n junction charge transport model, to 1 and 2 respectively. However, ηD and μr are only exceptionally equal to the above said values given by Schockley's model, moreover, there is no model to predict their actual experimental value. Even worst, should be identical diodes from one place to another in the same wafer, rarely have the same ηD and ηr values. The variation of each one of the above described p-n junction diode charge transport parameters introduce errors in temperature measurements, making necessary a, one to one, p-n junction thermometer calibration. Notwithstanding such drawbacks the p-n junction based thermometers and temperature sensors are widely used in countless applications.
With the aim to reduce the effect of some of the above mentioned problems constituting error sources present in pn junction based thermometers and temperature detectors several propositions have been made. For example, Thomson D. et al., in their American U.S. Pat. No. 6,554,469 propose, in an attempt to reduce the parasitic series resistance effect on the measured temperature value, a method using a transistor emitter-base p-n junction, to realize a set of four current measurements through this p-n junction, followed of some elaborated calculations in which the transistor current gain should be used. However, they neglect the effect of other p-n junction parameters that might introduce errors in the obtained temperature, as the value of the ideality factor, which might depend on the current intensity and variations on it because physical evolution of the device. Such inconvenient is completely absent in our method.
With similar purposes Matsuno Y. et. al., in their American U.S. Pat. No. 6,255,891 propose the use of two bipolar transistors of different emitter areas, as being part of an electronic circuit including comparators and a reference voltage, completely unnecessary in our hereby detailed method.